Motor control systems that drive industrial machines include a motor that supplies power to an object to be driven; a controller that generates a position command for the motor and transmits the position command to a motor control apparatus; an absolute-position detection apparatus that detects positional data on the motor and transmits the positional data to the motor control apparatus; and the motor control apparatus that drives the motor on the basis of the position command for the motor and the positional data on the motor.
A constant voltage needs to be supplied to the absolute-position detection apparatus for the absolute-position detection apparatus to detect and store the positional data. While power is supplied to the motor control apparatus, a constant voltage can be supplied to the absolute-position detection apparatus from a constant-voltage power supply circuit in the motor control apparatus. However, when power is not being supplied to the motor control apparatus, it is necessary to provide an auxiliary power supply to supply a constant voltage to the absolute-position detection apparatus.
A voltage supply apparatus that is a conventional auxiliary power supply uses a primary cell that is compact and does not need an external power supply. However, the output voltage of the primary cell drops when it is used over the long-term because of the product life and aging. The measure conventionally taken against this problem is that, when the output voltage of the primary cell drops, the absolute-position detection apparatus transmits a warning signal to warn that the output voltage of the primary cell has dropped or the motor control apparatus monitors the output voltage of the primary cell, thereby prompting the user to replace the primary cell. However, when the output voltage of the primary cell drops sharply, there is not sufficient time before the primary cell becomes unusable after the warning signal is generated, i.e., the cell replacement grace period for replacing the primary cell is not sufficient. This poses a problem in that the absolute-position detection apparatus cannot retain positional data and thus the positional data is lost.
To solve such a problem, the voltage supply apparatus in Patent Literature 1 described below includes a main backup cell that is a secondary cell and an auxiliary backup cell that is a primary cell. When the output voltage of the main backup cell drops below a constant value, the voltage supply apparatus switches the voltage supplied to the absolute-position detection apparatus from the output voltage of the main backup cell to the output voltage of the auxiliary backup cell.